1. Field of the Invention
The present invention relates to an apparatus and method for advancing signatures used in printing presses. In particular, the present invention relates to an apparatus and method for slowing signatures after folding which eliminates buckling and wrinkling.
2. Description of the Prior Art
In printing presses, it is known to process signatures after printing using a folding apparatus, to provide one or more folds in the printed signatures. After folding, it is desirable to slow the speed of the folded signatures and/or advance the folded signatures to other portions of the press for further processing.
Tail snubbers are a prior art mechanism which have been used to slow signatures emerging from a folding apparatus. Tail snubbers create a nip through some portion of their rotation, usually 90xc2x0, to thereby grasp a passing folded signature in the nip. Tail snubbers are designed to grasp a signature at its trailing or tail end, and have a linear speed which is slower than the speed of the signature. This slower speed of the tail snubbers causes the folded signature to slow down when it is grasped in the nip.
One disadvantage of the use of tail snubbers to slow and/or advance signatures is that they can cause buckling or wrinkling of the folded signatures. The buckling or wrinkling of the folded signatures results when a folded signature driven on a tape has its tail end grasped by the nip of a slower-moving tail snubber, resulting in two different speeds being applied to portions of the folded signatures. Buckling of the signatures can result in unwanted creasing or folding of the signatures, and can also result in jamming of the signatures in the press. In addition, tail snubbers are disadvantageous because they require that a lateral adjustment be made of the position of the tail snubber every time a new signature size is used, so that the tail snubber is positioned to grasp the tail end of the folded signature of a particular length.
The present invention is an apparatus and method for advancing and/or slowing signatures in a printing press which eliminates disadvantages in prior art signature-advancing and slowing mechanisms. The apparatus and method includes a series of two or more flexible mechanical drives, such as belt drives or chain drives, where each drive includes at least a pair of opposed belts. The belts are preferably timing or toothed belts driven by sprockets. The sprockets are formed with a partial out-of-round surface, preferably a semi-elliptical outer surface. Preferably, two sprockets are used for each belt or chain, and the sprockets both have a semi-elliptical outer surfaces driven in phase with one another, and also have a 1:1 diameter ratio. As a result of these features, the belts or chains have two directions of motion. The first direction of motion of the belts or chainsxe2x80x94horizontalxe2x80x94advances the signatures and may be used to slow the signatures. The second direction of motion of the belts or chainsxe2x80x94verticalxe2x80x94retracts the belts or chains away from engagement with the signatures. Retracting the belt from engagement with the signatures prevents buckling or wrinkling during a speed transition or during a transfer between belts.
One of the sprocket shafts may be fixed, while the other sprocket shaft may be movable or float, so that the tightness or tension of the belts may be adjusted. In one preferred embodiment, sprockets may be used which are fabricated from standard, circular timing belt sprockets which have had one side ground or otherwise machined to a semi-elliptical shape. The use of a semi-elliptical shape ensures that the pitch length of the belt remains constant throughout its movement through a complete cycle, and as a result, there is no change in tension in the belt. The design of the apparatus of the present invention therefore requires no mechanism to compensate for tension changes, which could cause unwanted vibrations. The teeth on one side of the sprockets positively drive the timing belts during a rotation, while slip occurs between the timing belts and the semi-elliptical side of the sprockets, from the velocity difference due to the changing radius.
The sprockets which drive the belts may in turn be driven by a driving mechanism, which can be in the form of a driven belt with its own tensioner. Other drive mechanisms, such as gears or motors, could also be used to drive the sprockets of the present invention.
In the method of the present invention, signatures are fed between two opposed belts, at least one of which is retractable, i.e., movable in two directions. The signatures are advanced by the belts during the one-half rotation of the sprockets at which the belts are in an extended or engaged position. During the other one-half of a rotation of the sprockets, at least one of the belts is retracted and disengaged from the signature, allowing the signature to be engaged by another pair of opposed belts without buckling or wrinkling caused by an engagement of an end of the signature with the first opposed belts.
The device of the present invention may be adapted to ensure that the speed at which the belts advance the signatures is optimal, and the rate at which the belt retracts or disengages from the signatures is also optimal. The size and shape of the sprockets will dictate these parameters. The advancing speed at which the signatures are driven is a function of the pitch diameter of the sprocket. The rate at which the belt retracts or disengages from the signatures will be a function of the semi-elliptical profile of the sprockets.
The sprockets driving a particular belt are arranged so that they are always in phase with one another, i.e., the toothed side on one sprocket is always facing in the same direction as the toothed side of any other sprocket for that belt. In this way, the tension in the belt is maintained, as the same number of sprocket teethxe2x80x94one-half of a sprocket circumferencexe2x80x94are engaged with the belt through the entire rotation of the sprockets. This ensures a positive drive of the belts, and thus a positive drive of the signatures, throughout the rotation of the sprockets without any change in the surface speed of the belts.
The major axis of the semi-elliptical surface on the sprockets is equal to the diameter of the sprocket measured from the bottom of the teeth of the sprocket. The minor axis of the semi-elliptical surface is calculated so that the arc length of the semi-elliptical surface is equal to a distance of any integer number of teeth on the belt. This arc length can be varied to any number which produces the desired amount of vertical lift of the belt which is required for the particular design or operating conditions.
In one embodiment of the present invention, both opposed belts are retracting belts; in another embodiment, one belt is a fixed conveyor belt, while the other opposed belt is a retracting belt. The apparatus can be formed of a series of sequential belts running at different speeds, thereby resulting in the speeding up or slowing down of the signatures as they pass from belt to belt. Alternatively, instead of using a series of sequential belts, the slower set of belts could be located inside the faster set of belts. In another embodiment, the upper and lower belts can be offset relative to one another to create an S-wrap along the signature, thereby compensating for different thicknesses of the folded signature.